In mechanical watches, regulating members, in particular escapements have to satisfy several “safety” criteria. One of the safety devices, the anti-trip system, is designed to prevent the angular extension of the balance beyond a normal angle of rotation.
The technical problem is to devise a safety mechanism, particularly an anti-trip system, which limits the angle of pivoting of a balance during excessive accelerations, in particular in the event of shocks, notably for a detent escapement. The anti-trip mechanism must be capable of acting in both directions of pivoting of the balance, i.e. both during extension and contraction of the balance spring.
One solution consists in changing the geometry of the balance spring by causing the lugs of consecutive coils to cooperate in abutment, so as to render some coils inactive and thus to modify the rigidity of the balance spring and its response to impulses. A mechanism of this type, capable of limiting the angular travel of the balance in both directions of pivoting is known from EP Patent No 2 434 353 A1 in the name of MONTRES BREGUET SA, which discloses an anti-trip balance spring wherein notches pertaining to consecutive coils cooperate with each other, both during the contraction and the expansion of the balance spring.
US Patent Application No 2009/116343A1 in the name of LEVINGSTON GIDEON describes a balance spring comprising, on the outer coil thereof, a balance spring stud including two lips which can enter into contact with the balance spring during extensions or contractions which are higher than normal, and thereby modify the active length thereof.
EP Patent Application No 1857891A1 in the name of PATEK PHILIPPE describes a one-piece balance spring-collet assembly, the outer contour of the collet comprising at least one stop member with which the balance spring cooperates in abutment in the event of a shock before exceeding the elastic limit of the inner coil.
EP Patent Application No 1818736A1 in the name of the SWATCH GROUP R&D describes a shock absorber collet whose asymmetrical contour follows the inner coil of the balance spring at a substantially constant distance, said contour being able to take the form of a curve provided with discrete bearing points.
U.S. Pat. No. 3,041,819A in the name of ENSIGN, GEORGE describes a sprung balance, wherein the arm of the balance bears a pin provided with a balanced strip, said strip being arranged, according to the angular orientation thereof relative to the arm, to exert a braking force to prevent tripping. In a variant, the outer coil of the balance spring bears a unit capable of interfering with a pin carried by an arm of the balance.
It is an object of the present invention to improve safety, while only very slightly disturbing the inertia of the balance, by limiting the angular travel thereof in both directions of rotation.